Film with piezoelectric polymer region

ABSTRACT

A film comprising a piezoelectric polymer has an upper surface and a lower surface. The film has an active region comprising the piezoelectric polymer, which extends from the upper surface of the film to the lower surface of the film. The film also comprises an adhesive sheet, which defines part of the upper or lower surface of the film. Circuit sheets may be bonded to the upper and lower surfaces in a lamination process to produce a laminated piezoelectric device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, U.S. application Ser. No. 16/340,614, filed Apr. 9, 2019,which is a national stage of and claims priority under 35 U.S.C. § 371to PCT Application No. PCT/GB2017/053040, filed on Oct. 6, 2017, whichclaims priority to Great British Application No. 1617171.2, filed onOct. 10, 2016. The contents of these priority applications are herebyincorporated by reference in their entirety.

This invention relates to laminated piezoelectric devices, and tocomponents and methods for manufacturing the same.

Piezoelectric materials generate a voltage when subjected to mechanicalstress, and have many applications such as in microphones and sensors.They can also deform when subjected to an externally-applied voltage,and so can be used as loudspeakers, motors, actuators, etc. Somepiezoelectric materials also exhibit a pyroelectric effect, enablingthem to be used for applications such as infrared detectors.Piezoelectric materials are used across a wide range of industries,including manufacturing, automotive, telecommunications and medicalinstruments.

Piezoelectric devices traditionally employ bulk crystal or ceramicmaterials that can exhibit a piezoelectric effect. More recently,however, polymers such as polyvinylidene fluoride (PVDF) and thecopolymer poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE))have increasingly been used. Unlike ceramics, these piezoelectricpolymers are flexible. They can also be processed using techniques suchas spraying, spin-coating, and dip-coating. For acoustic applications,they provide a good acoustic match to many other materials such aswater, biological tissue, and polymers. Similar advantages can also beprovided by piezo-composite materials, which incorporate piezo-ceramicparticles into a polymer (e.g., in powdered form), which is notnecessarily a piezoelectric polymer (e.g., an epoxy resin). For thepurposes of this patent application, piezoelectric ceramic-polymercomposites should be considered to be included within the general term“piezoelectric polymer”.

Piezoelectric polymers are well suited to micro-fabrication. Suchdevices can be made to be flexible, making them well suited for varioussensing applications, and for incorporating into flexible circuits.

Most of today's devices for piezoelectric sensors and transducers useconductive electrode materials that are attached directly to thepiezoelectric material, or that are attached to an external circuit.

An example of direct attachment is a commercially-available metallizedpiezo-film sheet, which consist of a PVDF film covered top and/or bottomwith metal, such as a compliant silver ink, or one or more sputteredmetals. These metal layers may be etched to create electrodes forconnection to a circuit, as described in “Flexible Piezoelectric TactileSensor Array for Dynamic Three-Axis Force Measurement” by Ping Yu etal., Sensors 16, no. 6: 819. However, such etching is complex andinvolves chemicals that are environmentally harmful. Connecting anelectronic circuit to the electrodes (e.g., by wire-bonding or aconductive adhesive) is not straightforward due to the temperaturelimitations of the PVDF material.

Alternatively, an external circuit can be attached to a piezoelectricmaterial during the manufacturing process. A thin dielectric adhesivelayer can be used to bond the piezoelectric material to the circuitmaterial or electrode. This adhesive layer has to be thin, in order toavoid a significant reduction of the electric field inside thepiezoelectric material. The adhesive may also need to match the acousticimpedance of the other materials. These requirements can be challengingto meet in a production process.

The paper “Properties and characteristics of P(VDF/TrFE) transducersmanufactured by a solution casting method for use in the MHz-rangeultrasound in air” by Takahashi, Ultrasonics 52 (2012) 422-426,identifies challenges of adhesive bonding. It describes an alternativeapproach in which P(VDF/TrFE) is cast directly onto an aluminium backingplate.

However, the present invention takes a different approach.

From a first aspect, the invention provides a film having an uppersurface and a lower surface, the film comprising (i) an active regioncomprising a piezoelectric polymer, the active region having a thicknessthat extends from the upper surface of the film to the lower surface ofthe film, and (ii) an adhesive sheet, wherein the adhesive sheet definespart of the upper or lower surface of the film.

From a second aspect, the invention provides a laminated devicecomprising a film laminated to a circuit sheet, wherein:

-   -   the film includes (i) an active region comprising a        piezoelectric polymer, the active region having a thickness that        extends from an upper surface of the film to a lower surface of        the film, and (ii) an adhesive sheet, wherein the adhesive sheet        defines part of the upper or lower surface of the film;    -   the circuit sheet comprises an electrode region adjacent the        active region of the film; and    -   the circuit sheet is bonded to the adhesive sheet of the film        outside the active region.

From a third aspect, the invention provides a method of manufacturing alaminated device comprising a film and a circuit sheet, wherein:

-   -   the film includes (i) an active region comprising a        piezoelectric polymer, the active region having a thickness that        extends from an upper surface of the film to a lower surface of        the film, and (ii) an adhesive sheet, wherein the adhesive sheet        defines part of the upper or lower surface of the film; and    -   the circuit sheet comprises an electrode region,        the method comprising laminating the circuit sheet to the film        by:    -   locating the electrode region of the circuit sheet adjacent the        active region of the film; and    -   bonding the circuit sheet to the adhesive sheet of the film        outside the active region.

Thus it will be seen by those skilled in the art that, in accordancewith the invention, a piezoelectric-capable film has an adhesive sheetalongside a piezoelectric-capable active region, within the thickness ofthe film. In some embodiments, one or more regions of piezoelectricpolymer may form “islands” within, and through, the adhesive sheet. Atleast one of the upper and lower surfaces of the film has an active areaand a separate adhesive area. This allows a circuit sheet to be bondedto the film without the need to apply any additional adhesive layerbetween the film and the circuit sheet. In particular, it enables anelectrode on the circuit sheet to be in direct contact with the activearea or region of the film, without any intervening adhesive layer. Thisensures a good and consistent electrical coupling between the electrodeand the active region. It can also provide a good acoustical couplingbetween the electrode and the active region, which may be equallyimportant for certain applications, such as microphones.

The adhesive sheet can also provide a support to the film, facilitatinghandling of the film. It may also define (on its own, or in combinationwith other elements) the thickness and size of the film, and of theactive region within the film.

The manufacturing method may comprise heating the active region, whenlaminating the circuit sheet to the film, although this is notessential. This can allow the active region to bond more strongly withthe electrode region, e.g., by causing the active region to melt, atleast partially, and deform towards the electrode region. Such heatingcan also improve the uniformity of the active region at an interfacewith the electrode region. The electrode region preferably touches theactive region, and is preferably bonded to it (albeit preferably notusing a traditionally-applied adhesive layer).

It will be understood that the piezoelectric polymer may need to bepolarised (e.g., by applying a strong electric field across at leastpart of the active region, under appropriate temperature conditions)before the active region will exhibit a piezoelectric effect. The term“piezoelectric”, as used herein, should therefore be understood asincluding all materials that are capable of giving a piezoelectriceffect after appropriate treatment, such as annealing and/or poling.

The film may have any appropriate thickness. However, in someembodiments the film has a thickness (a mean thickness, or a maximum orminimum thickness, or a thickness at one point), of between 1 micrometreand 1 millimetre, or between 5 micrometres and 500 micrometres—e.g.,around 50 micrometres. The film is preferably of substantially uniformthickness—for example, having a thickness that deviates by at most+/−1%, +/−10% or +/−50%—over at least 50% or 75% or 90% or 100% of thearea of the film. The upper and lower surfaces of the film may have anyshape; in some embodiments they may be rectangular. The film may be ofany appropriate size—for example, it may have a length of 1, 10 or 100centimetres or more, and it may have a width of 1, 10 or 100 centimetresor more. The film is preferably flexible. It may have a substantiallyplanar upper and/or lower surface when in a relaxed state. It will beappreciated that terms such as “upper”, “lower”, “length” and “width”are used for convenience only, and are not intended to limit the film tobeing used in a horizontal orientation. In some embodiments, thelaminated device is flexible.

The film may comprise other materials beyond the adhesive sheet and thepiezoelectric polymer. However, in a preferred set of embodiments, theadhesive sheet and the polymer together make up at least 50%, 75%, 90%or 100% of the film by mass, or by surface area on the upper or lowersurface of the film.

The active region may have any appropriate shape—e.g., having a square,rectangular or circular cross-section, within the plane of the film whenthe film is lying flat. The active region preferably extends from theupper surface to the lower surface of the film over at least 90% of theactive region, by surface area—and preferably over the whole area. Theactive region preferably defines an active area of the upper surface ofthe film; it preferably defines an active area of the lower surface ofthe film.

The active region is preferably surrounded by the adhesive sheet, for atleast part of the thickness of the film. This helps to ensure a reliableconnection with the circuit sheet. The adhesive sheet may also therebyprovide a frame for the active region, containing the active regionduring manufacturing and/or use of the film (e.g., when the activeregion is in a liquid phase). However, it is not essential that theadhesive sheet fully surround the active region.

The adhesive sheet may be substantially homogeneous in its composition,over its depth and/or area.

The adhesive sheet preferably touches the active region. The adhesivesheet may meet the active region at an edge face of the active regionwithin the film. This edge face may extend from the upper surface to thelower surface of the film, although this is not essential. The edge facemay be planar; it may be perpendicular to the upper or lower surface ofthe film.

In one set of embodiments, the adhesive sheet has a thickness thatextends from the upper surface of the film to the lower surface of thefilm. The adhesive sheet would thus define parts of both the uppersurface and the lower surface of the film. In such embodiments, theadhesive sheet may extend from the upper surface to the lower surface ofthe film over at least 90% and preferably the whole of the adhesivesheet—i.e., so that preferably the film consists only or substantiallyof the adhesive sheet over those areas of the film where the adhesivesheet is present.

Such a film may, for example, be manufactured by cutting or forming ahole through an adhesive sheet, and filling the hole with thepiezoelectric polymer, to the same thickness as the adhesive sheet.Alternatively, the adhesive sheet could be deposited or formed around aslab of the piezoelectric polymer. These methods of manufacturing a filmare further respective aspects of the invention.

In another set of embodiments, over some or all of the area of theadhesive sheet, the adhesive sheet does not extend from the uppersurface of the film to the lower surface of the film. This may beappropriate if it is desired to bond a circuit sheet only to one surfaceof the film. It can also be advantageous in thicker films (e.g., havinga thickness of greater than 50 micrometres) which may require additionalstructural support. Thus, in some embodiments, the film comprises asecond adhesive sheet that defines part of the lower or upper surface ofthe film, and further comprises a support sheet (e.g., a stiffeningsheet)—preferably of a non-adhesive material, and preferably a polymersheet—that lies between the second adhesive sheet and the first adhesivesheet. The support sheet may be present over the whole area occupied bythe first and/or second adhesive sheet, or only a portion of this area.The second adhesive sheet preferably touches the active region. It maymeet the active region at an edge face of the active region within thefilm. It may surround the active region. The first and second adhesivesheets preferably respectively define parts of the upper and lowersurfaces of the film.

The active region preferably comprises PVDF or a copolymer of PVDF, suchas P(VDF-TrFE). The active region may consist only of a piezoelectricpolymer, or only of a piezoelectric polymer and a coupling agent.However, in other embodiments the active region may comprise a mixtureof a piezoelectric polymer and at least one other material. Thepiezoelectric polymer may be, or may comprise, a piezoelectricceramic-polymer composite (such as PiezoPaint™ by Meggitt A/S).

Manufacturing the laminated device may comprise melting a mixture of apiezoelectric ceramic powder and the polymer.

The manufacturing method may also comprise poling at least part of theactive region to polarise the piezoelectric polymer. This may beachieved by applying an electric field across at least a part of theactive region and/or heating or cooling the active region, as is knownin the art. The electrode region may be used to apply the electricfield. It may also be necessary to treat the piezoelectric polymer toactivate a phase change—e.g., to change PVDF from an alpha phase to abeta phase. This is referred to as annealing. When using the copolymerP(VDF-TrFE), annealing may be carried out after lamination, by applyingheat and pressure for a defined period (e.g., 2-3 hours).

In some embodiments, the active region and/or the electrode region ofthe circuit sheet may be treated to increase a bonding strength betweenthe active region and the electrode region. Possible treatments includeplasma treatment, a chemical primer, or a combination of these. Themethod of manufacturing may therefore comprise applying a plasma or achemical primer to the active region or to the electrode region. Forinstance, the active region and/or the electrode region may be primedwith a coupling agent (also called a cross-linking agent), such as asilane. Such treatments can improve the electrical connection betweenthe active region and the electrode region. In some embodiment, theactive region may therefore further comprise a coupling agent. Thecoupling agent is preferably present at a surface of the active region.

It will be appreciated that priming a surface of the active region witha coupling agent, or treating the surface using a plasma, is verydifferent from adding a traditional adhesive layer onto a piezoelectricmaterial, as is known from the prior art. In particular, thecross-linking action of a coupling agent in the polymer of the activeregion is different from the chemical interaction of a typical adhesivelayer-relying on ionic bonding, rather than the Van de Walls forces ormechanical action used by a typical adhesive. A silane coupling agent,for instance, is a single molecule where one end is tailored to bondwell to the organic side (e.g., PVDF polymer) while the other side istailored to bond strongly to the inorganic side (e.g., an electrodematerial like Au). The coupling agent is therefore preferably depositedwith a thickness of only around one or two molecules thick. To achievesuch a monolayer, a solution, such as ethanol, is preferably used towash excess coupling agent from the active region or from the electroderegion of the circuit sheet.

The coupling agent is preferably much thinner than a typical adhesivelayer would be. Preferably, the surface treatment influences only a fewmolecular layers around an interface between the active region and theelectrode region—e.g. having a thickness of less than 10, 100 or 1,000nanometres in the laminated device. In particular, a coupling agentlayer preferably has a mean or maximum thickness, across the primedactive region, of no more than 10, 100, or 1,000 nanometres—for example,around 0.8 nm. Because any surface treatment is so thin, it will causeonly a small reduction to the electric field in the active region. Bycontrast, a traditional epoxy or acrylic adhesive needs to polymerizeinto very long molecules, which limits how thin such layers can be made.The way that such adhesives are typically applied also makes itdifficult to make thing, uniform layers (e.g., of less than a micron).

The adhesive sheet may comprise an epoxy, acrylic or polyimide adhesive.The adhesive may be only partially-cured before lamination. This hasbeen found to facilitate the forming of a void for filling with thepiezoelectric polymer during manufacture of the film. The adhesive sheetmay comprise Pyralux™ LF Sheet Adhesive from Dupont™.

The circuit sheet may comprise a polymer layer and a metal layer. Theelectrode region may be formed in the metal layer. It may besubstantially the same size and/or shape as the area of the activeregion on the upper or lower surface of the film, or it may be smalleror larger. A plurality of electrode regions in the circuit sheet may bebonded to the one active region. The circuit sheet may comprise anysuitable material. It may comprise a composite such as FR4, commonlyused in PCBs. However, in some embodiments, the circuit sheet maycomprise an isotropic polymer, preferably one with a high glasstransition temperature like polyimide (PI) or polyethyleneimine (PEI).These may be preferred due to their isotropic properties and/or lowsound attenuation.

The circuit sheet may comprise a polymer in-fill (e.g., comprising apolyimide adhesive or a high-temperature epoxy) adjacent or surroundingthe electrode region. The polymer in-fill may have a surface that isflush with a surface of the electrode region. The polymer in-fill andthe electrode region may have an equal thickness. The polymer in-fillmay be arranged to bond to the adhesive sheet when the circuit sheet islaminated to the film. Using a polymer in-fill to smooth the surface ofthe circuit sheet in this way can improve the bonding of the circuitsheet to the film.

The circuit sheet may be laminated to the upper or lower surface of thefilm. In some embodiments, the laminated device comprises a secondcircuit sheet, which may be laminated to the opposite surface of thefilm to the first circuit sheet. The second circuit sheet preferablycomprises a second electrode region that is adjacent the active regionof the film. The electrode regions of the first and/or second circuitsheets are preferably arranged for connection to one or more electricalcircuits, which may include any one or more of: a power supply, avoltage measuring system, an amplifier, an analogue driver, a DAC, anADC, a processor, a DSP, an ASIC, an FPGA, a resistor, a capacitor, awire, a socket, an antenna, etc. Some or all of these components may bepart of the laminated device (e.g., being part of, or connected to, oneor other of the circuit sheets), or they may be separate from thelaminated device.

The laminating of the circuit sheet to the film may comprise applyingheat and/or pressure to one or both of the circuit sheet and the film,at appropriate levels and for appropriate durations.

The film may comprise a plurality of active regions, each comprising apolymer that has a piezoelectric phase—e.g., 10 or 100 or more activeregions. Each of the active regions may have some or all of the sameoptional features as the active region described above. The activeregions are preferably spaced apart from each other within thefilm—e.g., separated by the adhesive sheet. The active regions may bearranged in a regular grid—e.g., a rectangular array. This is desirablewhen using the film as a beamforming transducer, such as an arraymicrophone or hydrophone, or an array speaker.

The laminated device may comprise a plurality of films as describedherein—e.g., stacked in layers with additional circuit layers inbetween. It may comprise additional circuit sheets, which may be bondedto the upper and/or lower circuit sheet, and which may be electricallyconnected to the upper and/or lower circuit sheet using vias.

The laminated device may be, or may form part of, a microphone, ahydrophone, a loudspeaker, an infrared sensor, an actuator, a voltagegenerator, a pressure sensor, a microbalance, an ultrasonic nozzle, anoscillator, or any other appropriate device. In a preferred set ofembodiments, the laminated device is a single or array microphone orloudspeaker—e.g., for receiving and/or transmitting ultrasound.

Using an adhesive-free interface between the piezoelectric material andthe electrodes located on external flexible circuits simplifiesconnection of electrical signals. Standard adhesive (bonding) materialscan be used outside the active piezoelectric domains. This is believedto have advantages in terms of, for example, increasing the overall bondstrength and simplifying connection between different circuit layers(e.g. by using known solutions for via connections). The adhesivematerials can be purchased in various thicknesses, which can then alsoact as “stoppers” for the lamination process, to control the finalthickness of the piezoelectric film. This can simplify the laminationprocess by not requiring external “stopping” parts to be used in thelamination tool, and by making control of the temperature and pressureparameters less critical.

Features of any aspect or embodiment described herein may, whereverappropriate, be applied to any other aspect or embodiment describedherein. Where reference is made to different embodiments or sets ofembodiments, it should be understood that these are not necessarilydistinct but may overlap.

Certain preferred embodiments of the invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a top view of a piezoelectric-capable film embodying theinvention;

FIG. 2 is an exploded cross-sectional side view of the film, incombination with an upper circuit sheet and a lower circuit sheet;

FIG. 3 is a bottom view of the upper circuit sheet;

FIG. 4 is a top view of the lower circuit sheet;

FIG. 5 is an exploded cross-sectional side view of a first varianthaving a film and two alternative circuit sheets; and

FIG. 6 is an exploded cross-sectional side view of a second varianthaving two circuit sheets and an alternative film.

The drawings are not to scale. In particular, the thickness (height) ofthe layers has been greatly exaggerated.

FIG. 1 shows a flexible, rectangle of film 1 in which a thin rectangleof polyvinylidene fluoride (PVDF) 2 is surrounded by an adhesive sheet3. The rectangle of film 1 shown in FIG. 1 may be the entire film, or itmay be part of a larger film, which may contain multiple PVDFrectangles—e.g., a regularly-spaced rectangular grid of such rectangles.

FIG. 2 shows a side view of the film 1, along the line A-A in FIG. 1.The rectangle of PVDF 2 and the adhesive sheet 3 have the same thicknessas each other, which is also the thickness of the film 1. This thicknessmay be from 5 to 50 micron. The film 1 may be fabricated by creating auniform rectangular sheet of partially-cured epoxy or acrylic adhesive,and then cutting the rectangle out—e.g., using milling, laser cutting,or die cutting—and replacing it with the PVDF 2.

A flexible upper circuit sheet 4 and a flexible lower circuit sheet 5are shown above the film 1 and below the film 1, respectively, lyingparallel with the film 1. These sheets 4, 5 are shown in a spaced-apartrelationship with the film 1, for ease of viewing. However, in reality,these circuit sheets 4, 5 will be placed in contact with the film 1prior to lamination, and will be bonded to the film 1 after lamination.

The upper circuit sheet 4 has a polymer layer 6, which may be polyimide(PI) or polyethyleneimine (PEI). A patterned metal layer has beendeposited or etched on the underside of the polymer layer. This metallayer includes a rectangular electrode 7, which is aligned with(although smaller than) the PVDF rectangle 2 in the film 1.

The lower circuit sheet 5 has a polymer layer 8, which may be PI or PEI.A patterned metal layer has been deposited or etched on the upper faceof the polymer layer. This metal layer includes a rectangular electrode9, which is aligned with the metal rectangle 7 of the upper circuitsheet 4.

FIG. 3 shows the underside of the upper circuit sheet 4. In addition tothe electrode 7, the patterned metal layer may provide other circuitfeatures such as a wire 10 for connecting the electrode 7 to anelectrical circuit which may include components that are located off thefilm 1.

FIG. 4 shows the top side of the lower circuit sheet 5. This maysimilarly include other circuit features such as a wire 11 forconnecting the electrode 9 to the same electrical circuit as theelectrode 7 on the upper circuit sheet 4.

During manufacture, the film 1 is sandwiched between the upper circuitsheet 4 and the lower circuit sheet 5, and heat and pressure areapplied—e.g., by a heated press or roller. This causes the polymerlayers 6, 8 of the circuit sheets 4, 5 to stick to the upper and lowerfaces, respectively, of the adhesive sheet 3 of the film 1, everywhereexcept adjacent the rectangle of PVDF 2. Instead, the electrodes 7, 9will contact the rectangle of PVDF 2 and will make physical andelectrical contact by a combination of mechanisms.

First, tension in the polymer layers 6, 8 of the circuit sheets 4, 5,once they are adhered to the adhesive sheet 3, will force the electrodes7, 9 onto the rectangle of PVDF 2.

Secondly, during lamination, sufficient heat is applied to the PVDF 2sufficient to cause it to melt or to near melting point; as the PVDF 2cools, it will conform closely to the surface of the electrodes 7, 9,providing good contact.

Thirdly, prior to lamination, the exposed surfaces of the electrodes 7,9 and/or the PVDF rectangle 2 may be treated with a plasma and/or achemical primer, such as a silane, to improve the bonding. This surfacetreatment preferably influences only a few molecular layers around theelectrode-PVDF interface, to minimize the electric field reduction inthe PVDF 2.

After lamination, the electrodes 7, 9 are used to apply an electricalfield across the rectangle of PVDF 2 in order to polarise the PVDF 2 soas to cause it to become piezoelectric. Thereafter, the laminated devicemay be used as, or incorporated into, a microphone, loudspeaker,actuator, or other component or circuit, as required.

In other embodiments, instead of PVDF 2, a different type ofpiezoelectric material may be used in the film 1, so long as it can beadapted to the temperature and pressure required for the bondingprocedure, and embedding between the flexible circuits 4, 5. Examples ofapplicable piezoelectric material are PVDF-TrFE), other copolymers ofPVDF, and composites of ceramic powder mixed with one or more polymers.The piezoelectric material could also have different initial forms whenit is inserted in a lamination press—for example, being a fluid or gelmixed with a solvent. It might instead be in the solid phase—e.g., inthe form of a pellet, a powder or a film. As already mentioned, duringthe lamination, the piezoelectric material should either be in a meltedphase or near melting at least during a part of the lamination cycle,where it can deform toward the flexible circuits 4, 5 and electrodes 7,9 to provide a sufficient strong bonding as well as uniformity to thefacing materials.

FIG. 5 shows a variant embodiment in which the same film 1 is sandwichedbetween alternative circuit sheets 4′, 5′. The upper circuit sheet 4′ isthe same as the upper circuit sheet 4 of FIG. 1 except that air gaps inthe patterned metal layer, including the rectangular electrode 7′, havebeen filled in with a filling material 12 to provide a planar lower faceto the upper circuit sheet 4′. This can improve the bonding to the upperface of the film 1. Similarly, the lower upper circuit sheet 5′ is thesame as the lower circuit sheet 5 of FIG. 1 except that air gaps in thepatterned metal layer, including the rectangular electrode 9′, have beenfilled in with a filling material 13 to provide a planar upper face tothe lower circuit sheet 5′.

FIG. 6 shows a variant embodiment in which an alternative film 14structure is sandwiched between the upper and lower circuit sheets 4, 5.This variant is particularly suitable where a thicker piezoelectricregion is desired—e.g., greater than 50 microns thick. A rectangle ofPVDF 2″ is surrounded by a three-layered structure comprising an upperadhesive sheet 15, a lower adhesive sheet 16, and—sandwiched betweenthese—a stiffening sheet 17 made of a suitable flexible polymer. Thefilm 14 may be formed by laminated complete rectangles of two adhesivesheets with a stiffening sheet, and then cutting out a rectangular hole,to be filled with PVDF 2″. The variant has additional stiffness, whichmay simplify handling of the film 14.

It will be appreciated by those skilled in the art that the inventionhas been illustrated by describing one or more specific embodimentsthereof, but is not limited to these embodiments; many variations andmodifications are possible, within the scope of the accompanying claims.

The invention claimed is:
 1. A laminated device comprising a filmlaminated to a circuit sheet, wherein: the film has an upper surface anda lower surface, the film comprising (i) an active region comprising apiezoelectric polymer, the active region having a thickness that extendsfrom the upper surface of the film to the lower surface of the film, and(ii) an adhesive sheet, wherein the adhesive sheet defines part of theupper or lower surface of the film; the circuit sheet comprises anelectrode region adjacent the active region of the film and a polymerin-fill adjacent or surrounding the electrode region; and the circuitsheet is bonded to the adhesive sheet of the film outside the activeregion.
 2. The laminated device as claimed in claim 1, wherein theactive region at least one of: extends from the upper surface to thelower surface of the film over the whole active region; and issurrounded by the adhesive sheet.
 3. The laminated device as claimed inclaim 1, wherein the adhesive sheet meets the active region at an edgeface of the active region within the film.
 4. The laminated device asclaimed in claim 1, wherein the adhesive sheet has a thickness thatextends from the upper surface of the film to the lower surface of thefilm.
 5. The laminated device as claimed in claim 1, wherein, over someor all of the area of the adhesive sheet, the adhesive sheet does notextend from the upper surface of the film to the lower surface of thefilm.
 6. The laminated device as claimed in claim 5, wherein the filmcomprises a second adhesive sheet that defines part of the lower orupper surface of the film, and further comprises a non-adhesive sheetthat lies between the second adhesive sheet and the first adhesivesheet.
 7. The laminated device as claimed in claim 6, wherein the secondadhesive sheet at least one of: meets the active region at an edge faceof the active region within the film; and surrounds the active region.8. The laminated device as claimed in claim 6, wherein the first andsecond adhesive sheets define parts of the upper and lower surfaces ofthe film, respectively.
 9. The laminated device as claimed in claim 1,wherein the active region comprises at least one of: PVDF or a copolymerof PVDF; and a piezoelectric ceramic-polymer composite.
 10. Thelaminated device as claimed in claim 1, wherein the adhesive sheet orsheets comprise an epoxy, acrylic or polyimide adhesive.
 11. Thelaminated device as claimed in claim 1, wherein the film comprises aplurality of spaced-apart active regions, each comprising apiezoelectric polymer.
 12. The laminated device as claimed in claim 1,wherein the circuit sheet comprises a polymer layer and a metal layer,with the electrode region formed in the metal layer.
 13. The laminateddevice as claimed in claim 12, wherein the polymer in-fill is providedadjacent the electrode region in the metal layer.
 14. The laminateddevice as claimed in claim 1, comprising a second circuit sheetlaminated to the film, with the film located between the first circuitsheet and the second circuit sheet.
 15. A method of manufacturing alaminated device as claimed in claim 1, the method comprising laminatingthe circuit sheet to the film by: locating the electrode region of thecircuit sheet adjacent the active region of the film; and bonding thecircuit sheet to the adhesive sheet of the film outside the activeregion.
 16. The method of manufacturing a laminated device as claimed inclaim 15, further comprising at least one of: heating the active regionso as to cause the active region to melt, at least partially, and soincrease bonding between the electrode region and the active region;poling and/or annealing at least part of the active region; applying aplasma to the active region and/or to the electrode region; priming theactive region and/or the electrode region with a coupling agent; andlaminating a second circuit sheet to the film, such that the film islocated between the first circuit sheet and the second circuit sheet.17. The laminated device as claimed in claim 1, wherein the polymerin-fill has a surface that is flush with a surface of the electroderegion, and/or wherein the polymer in-fill and the electrode region havean equal thickness.
 18. The laminated device as claimed in claim 1,wherein the polymer in-fill is arranged to bond to the adhesive sheetwhen the circuit sheet is laminated to the film.